Various methods are already known in the art for the manufacture of glass microspheres.
In accordance with the methods available in the art, pre-formed glass particles may be converted into glass bubbles or micro-balloons. To create the glass bubbles the glass particles have to be post-modified to contain the ingredients necessary for their expansion.
As known from the art, a silicate glass with over 14 wt % sodium oxide and 0.1 Fe2O3 is initially treated with high temperature steam and is used to form glass bubbles at about 1150-1200° C. In another known example, a silicate glass having a sodium oxide concentration of 13.5 wt % and a potassium oxide concentration of 3.2 wt %, with 0.2 wt % of sulfur in the form of SO4 is melted and pulverized. The so formed cullet is dropped through a flame at about 1150-1200° C. to create a glass microsphere.
In accordance with other methods known in the art of glass microsphere production, a pre-melted glass frit is used to produce glass micro-bubbles with a chemical composition, expressed in weight percent, of at least 67% SiO2 , 8-15% RO, 3-8% R2O, 2-6% B2O3, and 0.125-1.50% SO3. The RO: R2O weight ratio is in the range of 1.0-3.5.
From further art references, similar techniques to manufacture glass spheres are also known. These techniques employ a powdered glass with a glass composition comprising ranges of SiO2 of 50-57%, R2O of 2-15%, B2O3 of 0-20%, S of 0.05-1.5%, RO of 2-25%, R2O3 of 0-5%, and R2O5 of 0-5%. From further yet art references, the use of an essentially borosilicate glass composition is known to manufacture glass spheres having an oxide range of SiO2 of 60-90 wt %, an alkali metal oxide range of 2-20 wt %, a B2O3 range of 1-30 wt %, a sulfur range of 0.005 to 0.5 wt %, and other conventional glass-forming ingredients.
A particulate solid feed material having an average particle size of up to 25 microns is introduced at the top of a heating chamber into a “wall free” heated zone, according to a method known in the art for the manufacture of glass microspheres. The particles are transported to at least one flame front by a carrier gas comprising a fuel gas and air. The carrier gases maintain the particles in a dispersive state while the particles are heated to a temperature where at least partial fusion occurs, while the agglomeration of particles during fusion is inhibited. The resulting spherical particles are cooled and separated from the gas stream by a hot cyclone.
Yet another process for producing hollow microspheres know in the art comprises treating glass feed particles at a temperature above the working temperature of the glass. The particles are suspended in a gaseous current and passed through a burner for treatment. The particles are rapidly heated to about 1500 to 1700° C. for a residence time of less than about 0.1 seconds, and are cooled suddenly to below 750° C. The burner is operated such that the air factor, that is, the ratio of the amount of air introduced into the burner to the amount of air necessary to produce a stoichiometric combustion, is between about 0.75 and 1.1, or preferably 0.8-0.95. The particles are passed first through a reducing atmosphere and then through a non-reducing atmosphere.
Other known techniques for the manufacture of glass microspheres provide for a method in which feed material, in the form of solid glass particles, is introduced near the bottom of a furnace into an ascending column of hot gases. The feed material is entrained in an upward moving hot gaseous stream. The residence time of the particles within the furnace becomes a function of the mass of the particle, as the larger particles ascend through the hot zone of the furnace more slowly than the small particles due to the force of gravity acting on the particles. As a result, the residence time of the particles in the furnace is in direct relationship to the heat requirements necessary to expand the solid glass particles into hollow spheres.
Further technologies available in the art for manufacturing hollow microspheres employ a downward furnace suitable for heating discrete clay particles into hollow spherical particles. The particles are fed with a vibrating feeder into a hopper of a burner from where they are entrained in a stream of gas and passed through a flame front inside the furnace. The particles, in expanded state, are carried along with the flow of combustion gases and by gravity into a container. The container is at a sufficient distance from the combustion zone to provide a cooler zone, and solidification of the particles occurs before the particles hit any hard surfaces or before encountering each other to avoid agglomeration. A ratio of air to natural gas of about 2:1.1 was found to be suitable for raising the temperature of the particles to a range of 1350° C. to 1500° C., and to create the hollow microspheres.
A low cost method of converting solid glass or ceramic micro-particles into hollow microspheres, is also known in the art, and consists of feeding the solid glass or ceramic micro-particles, along with pulverized coal, into a coal-burning boiler. According to the known method coal-burning boilers generally produce micro-sized fused particles of ash, e.g. fly ash. A very small percentage of fly ash particles (about 1% and less) may be hollow, and these particles are commonly referred to as cenospheres. According to the known method the yield of hollow micro-particles is slightly increased by co-feeding fly or coal ash along with the pulverized coal.
Based at least on the above enumerated known methods and techniques, it is evident that there is still a need in the art for methods to manufacture hollow glass microspheres that have the following attributes:                high chemical durability in aqueous alkaline and acidic environments;        high crushing strength;        high hydrostatic pressure rating;        high specific strength;        are an eco friendly product, able to utilize in their make-up industrial waste byproducts, converting waste materials into highly value added products; and        are produced by sustainable (energy efficient) methods via fast manufacturing.        